Radiotherapy
Radiotherapy directs high-energy ionizing radiation at unhealthy tissues, e.g., a tumor, in the body of a patient while sparing healthy tissue. One form of radiation therapy is particle beam therapy, where a depth of a maximum exposure can be controlled. However, location of the tumors, especially tumors near vital organs, such the brain, liver, lung, stomach and heart, needs to be determined precisely. Therefore, it is desired to position all tissue structures of the patient in accordance with a treatment plan.
Radiotherapy directs high-energy ionizing radiation at unhealthy tissues in the human body while sparing healthy tissue. One form of radiation therapy is particle beam therapy, where a depth of a maximum exposure can be controlled. However, unlike in the traditional photon-based radiotherapy, the peak of the dose is located inside the tissue, and the exact position of the peak dose in depth is determined by the energy and the tissues in the path of the particle beam. Therefore, the location of target tissue needs to be determined precisely. Therefore, it is desired to exactly position all tissue structures of the patient in accordance with the geometric alignment of the treatment beam specified in the treatment plan.
Radiotherapy uses ionizing radiation as part of cancer treatment to control malignant cells. It may be used for curative or adjuvant cancer treatment. It is used as palliative treatment, where a cure is not possible and the aim is for local disease control or symptomatic relief, or as therapeutic treatment, where the therapy has survival benefit and it can be curative. Radiotherapy is used for the treatment of malignant tumors, and may be used as the primary therapy. It is also common to combine radiotherapy with surgery, chemotherapy, hormone therapy or combinations thereof.
In oncological cases, the radiation therapy is commonly applied primarily to the tumor. The radiation fields may also include adjacent lymph nodes if the nodes are clinically involved with the tumor, or if there is thought to be a risk of metastasis. It is necessary to include a margin of normal tissue around the tumor to allow for uncertainties in the set-up of the patient, and internal tumor motion.
It should be noted, that radiotherapy is typically provided in short sessions over several weeks, e.g., three or four, to allow the patient to recover between treatments. Thus, identical set-ups are difficult to achieve. Therefore, the patient's skin is usually marked with indelible ink, during treatment planning, to indicate to the radiotherapist how to set-up the patient relative to the beam.
The uncertainties in the set-up can also be caused by internal movement, for example, respiration and bladder filling, and movement of external skin marks relative to the tumor location.
To spare normal tissues, such as skin or organs, through which radiation must pass in order to treat the tumor, shaped radiation beams are aimed from several angles of exposure to intersect at the tumor, providing a much larger absorbed dose at the tumor than in the surrounding, healthy tissue. Typically, the radiation source is placed on a gantry that rotates around the patient. The goal is to place the tumor at the iso-center of the central axis of the beam as the beam rotates, so that the beam always passes through the tumor, and much less frequently through healthy tissue.
Positioning a Patient
A common problem in radiation therapy is positioning the patient with respect to radiation equipment in accordance to the treatment plan. The treatment plan is typically developed using a high-resolution computer tomography (CT) scan, which contains three-dimensional (3D) volume data representing a density of the tissue. During the treatment, the patient needs to be positioned with respect to the radiation equipment to ensure that the tumor is positioned at the iso-center of the central axis of the radiation beam and thus that the planned radiation dose is delivered to the tumor.
To achieve this goal, a set of X-rays is usually acquired and compared to the expected view of the CT volume. The error in the positioning is estimated and the patient is moved to the correct position. Currently this positioning is performed either manually or semi-automatically.
The manual method is tedious and requires an understanding of 3D geometry of objects manipulated over six-degrees of freedom (6-DOF). The therapist moves a couch with the patient and takes X-ray images after each movement. This procedure might take a long time and expose the patient to a large dose of non-therapeutic X-ray radiation.
Automatic methods may position the patient incorrectly. Therefore, the radiotherapist needs to check the results of the automatic positioning. Furthermore, the necessity of manipulating rendered 3D volumes by the therapist, for setting an initial condition or marking point-correspondences using conventional input devices, such as mouse or trackball, are difficult and unintuitive for the following reasons.
The CT scan data form a 3D volume that needs to be manipulated with 6-DOF. Such manipulation is difficult using a 2D manipulator (mouse) and a 2D image.
With conventional user interfaces, the mouse and a display device are not co-located, which makes it difficult to identify correspondences.